The present invention generally relates to rotation control apparatuses for controlling rotation frequencies of information recording discs, and more particularly to a rotation control apparatus for controlling a rotation frequency of a constant linear velocity system information recording disc such as a digital audio disc so that the information recording disc is rotated at a constant linear velocity.
In a constant linear velocity system (hereinafter simply referred to as a CLV system), a rotational speed of an information recording disc (hereinafter simply referred to as a disc) is controlled so that a linear velocity thereof is constant regardless of a scanning radial position on the disc. In other words, the rotational speed of the disc is controlled to become faster at radial positions toward the inner peripheral part of the disc and the rotational speed of the disc is controlled to become slower at radial positions toward the outer peripheral part of the disc.
In a reproducing mode of the CLV system, a signal reproduced from the disc by a signal detecting element is formed into a motor driving signal in a motor driving signal generating circuit, and this motor driving signal is supplied to a spindle motor which rotates a turntable on which the disc is placed.
On the other hand, in a recording mode of the CLV system, a position detecting circuit detects a radial position of a driving device which drives the signal detecting element, and this detected radial position is converted into a digital position information in an analog-to-digital converter. The digital position information is converted into a rotation frequency information in a control device such as a microcomputer. In the CLV system disc, the radial position of the driving device is inversely proportional to the rotation frequency (angular velocity) of the disc, and it is hence possible to obtain the rotation frequency information from the radial position information. The rotation frequency information obtained in the control device is converted into a voltage proportional to the rotation frequency in a digital-to-analog converter and is supplied to the spindle motor.
However, when newly recording an information on the disc from a position where a previous recording has been discontinued, the linear velocity during the new recording and the linear velocity during the previous recording may not accurately coincide due to effects of temperature changes and the like on the driving device. Thus, when the new recording is carried out with a linear velocity different from that during the previous recording, it is impossible to obtain in the reproducing mode a smooth reproduced signal from a connecting part on the disc where the previous recording ends and the new recording starts, and there is a problem in that the reproduced signal is unnatural at the connecting part. In addition, since the conventional rotation control apparatus comprises no means for detecting the present rotation frequency of the disc, it is impossible to satisfactorily control the rotation frequency of the disc when the operation mode is changed from the reproducing mode to the recording mode, for example.